“Not only is he going to be the Republican nominee, he offers a stark contrast to this president’s record.”

He brushed off concerns that Mr Romney had struggled to dominate the lengthy primary process, and said he had no doubt the former governor of moderate Massachusetts would campaign and govern as an authentic conservative.

“This primary has been excellent because it has meant people take position on issues that we can hold them accountable to,” he added.

‘Humble origins’

Mr Rubio’s endorsement is the latest high-profile Republican figure to fall in line behind Mr Romney, who is widely seen as the inevitable eventual nominee.

In a statement, Mr Romney said his latest backer was a living example of the American Dream.

“From humble origins, he has risen to become one of the brightest lights in our political party.